Laser absorption spectrometry

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Laser absorption spectrometry (LAS) refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry (AS).

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Optical spectroscopic techniques in general, and laser-based techniques in particular, have a great potential for detection and monitoring of constituents in gas phase. They combine a number of important properties, e.g. a high sensitivity and a high selectivity with non-intrusive and remote sensing capabilities. Laser absorption spectrometry has become the foremost used technique for quantitative assessments of atoms and molecules in gas phase. It is also a widely used technique for a variety of other applications, e.g. within the field of optical frequency metrology or in studies of light matter interactions. The most common technique is tunable diode laser absorption spectroscopy (TDLAS) which has become commercialized and is used for a variety of applications.

Direct laser absorption spectrometry

The most appealing advantages of LAS is its ability to provide absolute quantitative assessments of species. [1] Its biggest disadvantage is that it relies on a measurement of a small change in power from a high level; any noise introduced by the light source or the transmission through the optical system will deteriorate the sensitivity of the technique. Direct laser absorption spectrometric (DLAS) techniques are therefore often limited to detection of absorbance ~10−3, which is far away from the theoretical shot noise level, which for a single pass DAS technique is in the 10−7 – 10−8 range. This detection limit is insufficient for many types of applications.

The detection limit can be improved by (1) reducing the noise, (2) using transitions with larger transition strengths or (3) increasing the effective path length. The first can be achieved by the use of a modulation technique, the second can be obtained by using transitions in unconventional wavelength regions, whereas the third by using external cavities.

Modulated techniques

Modulation techniques make use of the fact that technical noise usually decreases with increasing frequency (often referred to as a 1/f noise) and improves on the signal contrast by encoding and detecting the absorption signal at a high frequency, where the noise level is low. The most common modulation techniques, wavelength modulation spectroscopy (WMS) [2] and frequency modulation spectroscopy (FMS), [3] achieve this by rapidly scanning the frequency of the light across the absorbing transition. Both techniques have the advantage that the demodulated signal is low in the absence of absorbers but they are also limited by residual amplitude modulation, either from the laser or from multiple reflections in the optical system (etalon effects). The most frequently used laser-based technique for environmental investigations and process control applications is based upon diode lasers and WMS (typically referred to as TDLAS). [4] [5] The typical sensitivity of WMS and FMS techniques is in the 10−5 range.

Due to their good tunability and long lifetime (> 10,000 hours), most practical laser-based absorption spectroscopy is performed today by distributed feedback diode lasers emitting in the 760  nm – 16 μm range. This gives rise to systems that can run unattended for thousands of hours with minimum maintenance.

Laser absorption spectrometry using fundamental vibrational or electronic transitions

The second way of improving the detection limit of LAS is to employ transitions with larger line strength, either in the fundamental vibrational band or electronic transitions. The former, which normally reside at ~5 μm, have line strengths that are ~2–3 orders of magnitude higher than those of typical overtone transition. On the other hand, electronic transitions have often yet another 1–2 orders of magnitude larger line strengths. The transitions strengths for the electronic transitions of NO[ clarification needed ], which are located in the UV range (at ~227 nm) are ~2 orders of magnitude larger than those in the MIR region.[ citation needed ]

The recent development of quantum cascade (QC) lasers working in the MIR region has opened up new possibilities for sensitive detection of molecular species on their fundamental vibrational bands. It is more difficult to generate stable cw light addressing electronic transitions, since these often lie in the UV region.

Cavity enhanced absorption spectrometry

The third way of improving the sensitivity of LAS is to increase the path length. This can be obtained by placing the species inside a cavity in which the light bounces back and forth many times, whereby the interaction length can be increased considerably. This has led to a group of techniques denoted as cavity enhanced AS (CEAS). The cavity can either be placed inside the laser, giving rise to intracavity AS, or outside, when it is referred to as an external cavity. Although the former technique can provide a high sensitivity, its practical applicability is limited by non-linear processes.

External cavities can either be of multi-pass type, i.e. Herriott or White cells, or be of resonant type, most often working as a Fabry–Pérot (FP) etalon. Whereas the multi-pass cells typically can provide an enhanced interaction length of up to ~2 orders of magnitude, the resonant cavities can provide a much larger path length enhancement, in the order of the finesse of the cavity, F, which for a balanced cavity with high reflecting mirrors with reflectivities of ~99.99–99.999% can be ~104 to 105.

A problem with resonant cavities is that a high finesse cavity has narrow cavity modes, often in the low kHz range. Since cw lasers often have free-running line-widths in the MHz range, and pulsed even larger, it is difficult to couple laser light effectively into a high finesse cavity. However, there are a few ways this can be achieved. One such method is Vernier Spectroscopy, which employs a frequency comb laser to excite many cavity modes simultaneously and allows for a highly parallel measurement of trace gases.

Cavity ring-down spectroscopy

In cavity ring-down spectroscopy (CRDS) the mode-matching condition is circumvented by injecting a short light pulse in the cavity. The absorbance is assessed by comparing the cavity decay times of the pulse as it "leaks out" of the cavity on and off-resonance, respectively. While independent of laser amplitude noise, this technique is often limited by drifts in the system between two consecutive measurements and a low transmission through the cavity. Despite this, sensitivities in the ~10−7 range can routinely be obtained (although the most complex setups can reach below this~10−9). CRDS has therefore started to become a standard technique for sensitive trace gas analysis under a variety of conditions. In addition, CRDS is now an effective method for different physical parameters (such as temperature, pressure, strain) sensing. [6]

Integrated cavity output spectroscopy

Integrated cavity output spectroscopy (ICOS) sometimes called as cavity-enhanced absorption spectroscopy (CEAS) records the integrated intensity behind one of the cavity mirrors, while the laser is repeatedly swept across one or several cavity modes.[ citation needed ] However, for high finesse cavities the ratio of "on" and "off" a cavity mode is small, given by the inverse of the finesse, whereby the transmission as well as the integrated absorption becomes small. Off-axis ICOS (OA-ICOS) improves on this by coupling the laser light into the cavity from an angle with respect to the main axis so as to not interact with a high density of transverse modes. Although intensity fluctuations are lower than direct on-axis ICOS, the technique is, however, still limited by a low transmission and intensity fluctuations due to partly excitation of high order transverse modes, and can again typically reach sensitivities ~10−7 .

Continuous wave cavity enhanced absorption spectrometry

The group of CEAS techniques that has the largest potential to improve is that based on a continuous coupling of laser light into the cavity. This requires however an active locking of the laser to one of the cavity modes. There are two ways in which this can be done, either by optical or electronic feedback. Optical feedback (OF) locking, originally developed by Romanini et al. for cw-CRDS, [7] uses the optical feedback from the cavity to lock the laser to the cavity while the laser is slowly scanned across the profile (OF-CEAS). In this case, the cavity needs to have a V-shape in order to avoid OF from the incoupling mirror. OF-CEAS is capable of reaching sensitivities ~10−8 range, limited by a fluctuating feedback efficiency. [8] Electronic locking is usually realized with the Pound-Drever-Hall (PDH) technique, [9] and is nowadays a well established technique, although it can be difficult to achieve for some types of lasers. [10] [11] It has been shown by that also electronically locked CEAS can be used for sensitive AS on overtone lines. [12] [13] [14]

Noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy

However, all attempts to directly combine CEAS with a locking approach (DCEAS) have one thing in common; they do not manage to use the full power of the cavity, i.e. to reach LODs close to the (multi-pass) shot-noise level, which is roughly 2F/π times below that of DAS and can be down to ~10−13. The reason is twofold: (i) any remaining frequency noise of the laser relative to the cavity mode will, due to the narrow cavity mode, be directly converted to amplitude noise in the transmitted light, thereby impairing the sensitivity; and (ii) none of these techniques makes use of any modulation technique, wherefore they still suffer from the 1/f noise in the system. There is, however, one technique that so far has succeeded in making full use of the cavity by combining locked CEAS with FMS so as to circumvent both of these problems: Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS). The first and so far ultimate realization of this technique, performed for frequency standard applications, reached an astonishing LODs of 5•10−13 (1•10−14 cm−1). [15] It is clear that this technique, correctly developed, has a larger potential than any other technique for trace gas analysis. [16]

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<span class="mw-page-title-main">Optical amplifier</span> Device that amplifies an optical signal

An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiber-optic cables which carry much of the world's telecommunication links.

<span class="mw-page-title-main">Raman spectroscopy</span> Spectroscopic technique

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<span class="mw-page-title-main">Absorption spectroscopy</span> Spectroscopic techniques that measure the absorption of radiation

Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a function of frequency, and this variation is the absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum.

Tunable diode laser absorption spectroscopy is a technique for measuring the concentration of certain species such as methane, water vapor and many more, in a gaseous mixture using tunable diode lasers and laser absorption spectrometry. The advantage of TDLAS over other techniques for concentration measurement is its ability to achieve very low detection limits. Apart from concentration, it is also possible to determine the temperature, pressure, velocity and mass flux of the gas under observation. TDLAS is by far the most common laser based absorption technique for quantitative assessments of species in gas phase.

Cavity ring-down spectroscopy (CRDS) is a highly sensitive optical spectroscopic technique that enables measurement of absolute optical extinction by samples that scatter and absorb light. It has been widely used to study gaseous samples which absorb light at specific wavelengths, and in turn to determine mole fractions down to the parts per trillion level. The technique is also known as cavity ring-down laser absorption spectroscopy (CRLAS).

<span class="mw-page-title-main">Tunable laser</span>

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Noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS) is an ultra-sensitive laser-based absorption technique that utilizes laser light to assess the concentration or the amount of a species in gas phase by absorption spectrometry (AS).

Gas in scattering media absorption spectroscopy (GASMAS) is an optical technique for sensing and analysis of gas located within porous and highly scattering solids, e.g. powders, ceramics, wood, fruit, translucent packages, pharmaceutical tablets, foams, human paranasal sinuses etc. It was introduced in 2001 by Prof. Sune Svanberg and co-workers at Lund University (Sweden). The technique is related to conventional high-resolution laser spectroscopy for sensing and spectroscopy of gas, but the fact that the gas here is "hidden" inside solid materials give rise to important differences.

Self-mixing or back-injection laser interferometry is an interferometric technique in which a part of the light reflected by a vibrating target is reflected into the laser cavity, causing a modulation both in amplitude and in frequency of the emitted optical beam. In this way, the laser becomes sensitive to the distance traveled by the reflected beam thus becoming a distance, speed or vibration sensor. The advantage compared to a traditional measurement system is a lower cost thanks to the absence of collimation optics and external photodiodes.

Incoherent broad band cavity enhanced absorption spectroscopy (IBBCEAS), sometimes called broadband cavity enhanced extinction spectroscopy (IBBCEES), measures the transmission of light intensity through a stable optical cavity consisting of high reflectance mirrors (typically R>99.9%). The technique is realized using incoherent sources of radiation e.g. Xenon arc lamps, LEDs or supercontinuum (SC) lasers, hence the name.

Vernier spectroscopy is a type of cavity enhanced laser absorption spectroscopy that is especially sensitive to trace gases. The method uses a frequency comb laser combined with a high finesse optical cavity to produce an absorption spectrum in a highly parallel manner. The method is also capable of detecting trace gases in very low concentration due to the enhancement effect of the optical resonator on the effective optical path length.

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References

  1. ↑ A. Fried and D. Richter: Infrared absorption Spectroscopy, in Analytical Techniques for Atmospheric Measurements (Blackwell Publishing, 2006)
  2. ↑ Kluczynski, Pawel; Gustafsson, Jörgen; Lindberg, Åsa M.; Axner, Ove (2001). "Wavelength modulation absorption spectrometry — an extensive scrutiny of the generation of signals". Spectrochimica Acta Part B: Atomic Spectroscopy. 56 (8): 1277–1354. Bibcode:2001AcSpe..56.1277K. doi:10.1016/S0584-8547(01)00248-8. ISSN   0584-8547.
  3. ↑ Bjorklund, G. C.; Levenson, M. D.; Lenth, W.; Ortiz, C. (1983). "Frequency modulation (FM) spectroscopy". Applied Physics B: Photophysics and Laser Chemistry. 32 (3): 145–152. Bibcode:1983ApPhB..32..145B. doi:10.1007/BF00688820. hdl:10261/57307. ISSN   0721-7269. S2CID   117556046.
  4. ↑ Cassidy, D. T.; Reid, J. (1982). "Atmospheric pressure monitoring of trace gases using tunable diode lasers". Applied Optics. 21 (7): 1185–90. Bibcode:1982ApOpt..21.1185C. doi:10.1364/AO.21.001185. ISSN   0003-6935. PMID   20389829.
  5. ↑ P. Werle, F. Slemr, K. Maurer, R. Kormann, R. Mucke, and B. Janker, "Near- and mid-infrared laser-optical sensors for gas analysis", Opt. Las. Eng.37 (2–3), 101–114 (2002).
  6. ↑ Paldus, Barbara A; Kachanov, Alexander A (2005). "An historical overview of cavity-enhanced methods". Canadian Journal of Physics. 83 (10): 975–999. Bibcode:2005CaJPh..83..975P. doi:10.1139/p05-054. ISSN   0008-4204.
  7. ↑ D. Romanini, A. A. Kachanav, J. Morville, and M. Chenevier, Proc. SPIE EUROPTO (Ser. Environmental Sensing)3821 (8), 94 (1999)
  8. ↑ Morville, J.; Kassi, S.; Chenevier, M.; Romanini, D. (2005). "Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking". Applied Physics B: Lasers and Optics. 80 (8): 1027–1038. doi:10.1007/s00340-005-1828-z.
  9. ↑ Drever, R. W. P.; Hall, J. L.; Kowalski, F. V.; Hough, J.; Ford, G. M.; Munley, A. J.; Ward, H. (1983). "Laser phase and frequency stabilization using an optical resonator". Applied Physics B: Photophysics and Laser Chemistry. 31 (2): 97–105. doi:10.1007/BF00702605.
  10. ↑ R. W. Fox, C. W. Oates, and L. W. Hollberg, "Stabilizing diode lasers to high finesse cavities", in Cavity-Enhanced Spectroscopies, R. D. van Zee and J. P. Looney, eds. (Elsevier Science, New York, 2002)
  11. ↑ Hall, J. L.; Hänsch, T. W. (1984). "External dye-laser frequency stabilizer". Optics Letters. 9 (11): 502–504. doi:10.1364/OL.9.000502.
  12. ↑ Nakagawa, K.; Katsuda, T.; Shelkovnikov, A.S.; de Labachelerie, M.; Ohtsu, M. (1994). "Highly sensitive detection of molecular absorption using a high finesse optical cavity". Optics Communications. 107 (5–6): 369–372. doi:10.1016/0030-4018(94)90349-2.
  13. ↑ de Labachelerie, M.; Nakagawa, K.; Ohtsu, M. (June 1, 1994). "Ultranarrow 13C2H2 saturated-absorption lines at 15 μm". Optics Letters. 19 (11): 840–842. doi:10.1364/OL.19.000840.
  14. ↑ Gagliardi, G.; Rusciano, G.; Gianfrani, L. (2000). "Sub-Doppler spectroscopy of H218O at 1.4 μm:". Applied Physics B: Lasers and Optics. 70 (6): 883–888. doi:10.1007/PL00021149.
  15. ↑ Ma, Long-Sheng; Ye, Jun; Dubé, Pierre; Hall, John L. (1999). "Ultrasensitive frequency-modulation spectroscopy enhanced by a high-finesse optical cavity: theory and application to overtone transitions of C2H2 and C2HD". Journal of the Optical Society of America B. 16 (12): 2255–2268. Bibcode:1999JOSAB..16.2255M. doi:10.1364/JOSAB.16.002255. L. S. Ma, J. Ye, P. Dube, and J. L. Hall, "Ultrasensitive frequency-modulation spectroscopy enhanced by a high-finesse optical cavity: theory and application to overtone transitions of C2H2 and C2HD", Journal of the Optical Society of America B-Optical Physics16 (12), 2255–2268 (1999)
  16. ↑ Foltynowicz, A.; Schmidt, F.M.; Ma, W.; Axner, O. (2008). "Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy: Current status and future potential". Applied Physics B. 92 (3): 313–326. Bibcode:2008ApPhB..92..313F. doi:10.1007/s00340-008-3126-z.
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